The invention relates to an activated sludge process for sewage purification in which the sewage is introduced into a first, partially aerated activation stage, is then submitted to intermediate clarification, is then introduced into a second, partially aerated activation stage, is subjected to post-clarification and is then removed, in which sludge is fed back from the intermediate clarification into the first activation stage and from the post-clarification into the second activation stage, and in which excess sludge is removed from at least one activation stage is removed from the sludge circuit.
The usual two-stage activated sludge process (Doctor W. Lindner, "The Two-Stage Activation Process in Sewage Purification" (Kempten 1957), Thomas-Verlag) is a known process. In this process, substrate breathing where the micro-organisms consume oxygen through the oxidation of organic compounds and where the biological decomposition of carbon compounds is therefore most conspicuous takes full effect in the first activation stage with high sludge load. The second activation stage in this known process is generally carried out with lower sludge load so that a decomposition of the remaining carbon compounds and the oxidation of nitrate compounds (nitrification) takes place.
Normally the known, two-stage activation process is carried out with a sludge proportion of 0.8 to 2.0 kg BO.sub.5 (biochemical oxygen requirement in 5 days)/kg dry substances and per day in the first stage and from 0.15 to 0.5 kg BOR.sub.5 /kg dry substance and per day in the second activation stage (Lehr- und Handbuch der Abwassertechnik [Instructions and Handbook for Sewage Technology], published by the Abwassertechnischen Vereinigung (Sewage Technology Association) e.V. in St. Augustin, Berlin Ernst Verlag, page 426, vol. IV, 3rd edition, 1985). In the method which is typical for this process, the major part of carbon decomposition takes place as described in the first stage, and extensive nitrification in the second stage. Denitrification is no longer possible in the second stage due to the absence of easily decomposed carbon substrate. The withdrawal of excess sludge from the overall plant takes place either during pre-clarification before the first stage or, in the absence of preclarification, from the sludge circuit of the first stage The excess sludge of the second stage is withdrawn together with the sludge of the first stage without giving rise to a controlled removal of nitrogen because no anoxic conditions exist in the first stage.
In a process known from AT-PS 318.503 for the elimination of organically and inorganically attached nitrogen from domestic and industrial sewage, nitrified sewage is taken from the sedimentation basin of the second process stage and is fed into a conduit through which sewage is conveyed from the aeration basin of the first stage into the post-clarification basin of the first stage for the denitrification of the ammonium nitrified in the second process stage, and thereby for the elimination of nitric nitrogen from the sewage to be purified. In the process known from AT-PS 318.503 only denitrification takes place in the first process stage (aeration basin and sedimentation basin) and only nitrification takes place in the second process stage.
In the process known from DE-OS 31 36 409, two plants, i.e. two stages of equal rank are parallel-connected for nitrification. According to the DE-OS 31 36 409 particularly favorable conditions for the nitrification are to be maintained in the auxiliary nitrification stage so that no interruption of nitrification may occur in case of disturbances in the main nitrification stage but that biology held in reserve in the auxiliary nitrification stage may be introduced from same into the main stage if necessary to ensure that nitrification can be continued therein substantially without interruption once a disturbance has occurred.
WO 83/00856 describes a sludge treatment process in which only carbon compounds are to be decomposed and in which neither nitrification nor denitirification takes place. In this process treated sludge coming from a pre-thickener is fed to an aerobic treatment stage for aerobic treatment. The discharge from the aerobic treatment stage is conveyed to an anaerobic treatment stage for a digestion process in which additional decomposition processes occur. Part of the substrate is fed back from the anaerobic treatment stage into the aerobic treatment stage according to WO 83/00856. No mixed biocenoses are produced thereby, since the bacteria which are active in the aerobic treatment stage are not viable in the anaerobic treatment stage and vice versa.
A process known under by the name "AB Process" from DE-PS 26-40-875 is a two-stage sewage purification process in which the first stage, an adsorption stage, is heavily charged (sludge proportion 2.0 to 10.0 kg BOR.sub.5 /kg dry substance and per day) and serves for the decomposition or adsorption of carbon compounds. The second stage operates with a sludge proportion of 0.15 to 0.30 kg BOR.sub.5 /kg dry substance and per day and serves for nitrification. Denitrification is possible only in the second stage in this process, and then only in function of the integration of carbon compounds not eliminated in the first stage, and this is difficult to control in operation. In the process known from DE-PS 25 40 875 the biocenoses of the first and second stage must be kept strictly separate from each other in order to provide the advantage of removal of carbon compounds in the first stage with little energy consumption and with adsorption.
The purification of sewage through nitrification and denitrification in activation plants can in carried out in principle in single-stage and in two stage plants. In order to achieve certain nitrification it is necessary to respect a given sludge age in the plant in function of temperature of the sewage so that the slowly growing nitrifying bacteria in the activated sludge may indeed be present and are not washed out. The age of the sludge in an activation plant is however decisively influenced by the proportion of carbon compounds in the sewage since the nitrifying bacteria represent only a small part of the entire biomass (less than 5%).
Due to these conditions the size of the single-stage activation plant for sewage purification through nitrification is mainly determined by the proportion of decomposable carbon compounds.
In two-stage processes a good basis for nitrification in the second stage can be created through the extensive decomposition of the carbon compounds in the first stage without nitrification and the subsequent treatment of the pre-purified sewage in the second stage because it is possible to operate with comparatively small basin volumes after removal of the carbon compounds of old-age sludge.
For the removal of nitrogen compounds from the sewage the reduction of nitrate into gaseous nitrogen (denitrification) taking place in the absence of dissolved, i.e. free oxygen and with the utilization of the oxygen attached to the nitrate (anoxic conditions) is however advantageous after the oxidation of nitrogen compounds (NH.sub.4 +) reduced to nitrate. In single-stage plants this can be achieved with appropriate configuration of the basin and adaptation of the oxygen arrival, so that aerobic and anoxic conditions are created. The inclusion of denitrification in the process requires however additional enlargement of the required basin volume.
The inclusion of denitrification in two-stage installation is basically possible, but because of the spatial separation between the decomposition of the carbon compounds in the first stage and the nitrification in the second stage, the supply of carbon compounds required for the reduction of the produced nitrate during denitrification is now very low. Effective denitrification in conventional two-stage processes is possible only through recirculation of nitrified, essentially sludge-free discharge from the post-clarification in the fist stage and the establishment of anoxic conditions in same (see also Wilhelm v.d. Emde "Betriebsweise von Belebungsanlagen" [Operation of activation plants] in Wiener Mitteilungen Wasser - Abwasser - Gewasser, Vol. 81, Vienna 1990, second edition).
The denitrification in the two-stage AB process (DE-PS 26 40 875) can only be achieved through limitation of the carbon removal in the first high-load stage, e.g. by reducing air supply, and thereby by shifting the carbon removal into the second stage, also while maintaining anoxic conditions. Recirculation before completion of the second stage into the first stage which is a highest-load stage would serve little purpose because of the short sojourn time. On the other hand however, the advantage with respect to maintaining optimal conditions in nitrification is lost because of the limitation of carbon removal in the first stage.
To sum up it can therefore be said for the operation methods used at this time in activation processes that in the single-stage process the production of a mixed biocenosis for carbon decomposition, nitrification and denitrification creates good conditions for the obtention of good results, but the volume requirements for activation basins for this are relatively high, e.g. 200 l/EGW (liter/population equivalence). In the two-stage processes the conditions for nitrification are generally better in the second stage because of the decomposition of the carbon compounds in the first stage. For denitrification however, considerable streams of volume must be fed back, and this leads to a heavy load for the sedimentation basin following the activation basin of the first or of the second stage. A reduction of the decomposition effect of the first stage (in the AB process) on the other hand represents a partial elimination of the advantages with respect to the nitrification in the second stage and can be achieved only at high operating costs with a reduction of the oxygen supply and the danger of odor emissions.